Field of the Invention
The present invention relates to a gas sensor that determines the concentration of a predetermined gas component such as a hydrocarbon gas in, for example, a measurement gas being an exhaust gas from a diesel engine.
Description of the Background Art
Gas sensors that sense a predetermined gas component in a measurement gas and determine its concentration come in various types. For example, a limiting current gas sensor and a mixed-potential gas sensor are well known (for example, see Japanese Patent No. 3566089 and Japanese translation of PCT international application publication No. 2002-540400).
Japanese Patent No. 3566089 discloses a limiting current gas sensor having a two-chamber structure including a first processing chamber into which a measurement gas is introduced through a first gas distribution part and a second processing chamber into which the gas of the first processing chamber is introduced through a second gas distribution part. The gas sensor is configured to adjust the concentration of oxygen (in practice, oxygen partial pressure) in the first processing chamber to a predetermined range (10−12 atm to 10−6 atm) that causes substantially no decomposition reaction of water vapor contained in the measurement gas and to determine the concentration of an inflammable gas component of the gas introduced into the second processing chamber on the basis of an amount of oxygen which is consumed when the inflammable gas component is burned. The gas sensor thus has the accuracy of sensing an inflammable gas, which is hardly affected by the decomposition of water vapor, and also can suitably operate in the lean-burn conditions.
Japanese translation of PCT international application publication No. 2002-540400 discloses a gas sensor including a sensor element, which can be used as a NOx sensor as well as a hydrocarbon (HC) sensor. The gas sensor includes a first measurement gas chamber provided with a first internal electrode catalytically inactive and a second internal electrode catalytically active, which is in contact with a measurement gas, a second measurement gas chamber separated from the first measurement gas chamber by an oxygen ion permeable layer, and a reference channel into which the air is introduced. The oxygen ion permeable layer includes a connection channel connecting the first measurement gas chamber and the second measurement gas chamber. Such a gas sensor conceivably functions as a mixed-potential gas sensor when used as an HC sensor.
In response to more stringent regulations on exhaust gas, there have recently been increasing demands for a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (TWC: three-way catalyst) of a gasoline engine and a diagnosis of failure in the performance of cleaning unburned hydrocarbon in an exhaust emission control system (DOC: diesel oxidation catalyst) of a diesel engine. These diagnoses require a gas sensor capable of sensing an unburned hydrocarbon gas and identifying its concentration.
For the gas sensor disclosed in Japanese Patent No. 3566089, a measurement gas having the lean composition, whose oxygen concentration has been reduced through oxygen pumping in the first processing chamber, is caused to have a stoichiometric composition or a somewhat rich composition and is then introduced into the second processing chamber. Although Japanese Patent No. 3566089 describes that such a configuration increases an output of the electromotive force of a concentration cell and also provides good sensitivity of the gas sensor, it involves such a problem that varying oxygen concentrations greatly affect the electromotive force of the concentration cell in the neighborhood of stoichiometric composition.
The gas sensor disclosed in Japanese translation of PCT international application publication No. 2002-540400 involves, for example, the following problems: because of its structure, errors tend to occur in reference oxygen concentration and control responsiveness of oxygen concentration becomes poor, and the positional relationship between a heater and each electrode does not reflect a temperature suitable for the operation of each electrode. Besides, Japanese translation of PCT international application publication No. 2002-540400 does not specifically describe how to control an oxygen concentration when the gas sensor is used as an HC sensor.